EFFECTS OF RAPTORS ON THE ACTIVITY OF SANDGROUSE

6 Pages·1994·2.2 MB·English

by  P N Ferns

,
J Raptor Res. 28(4);236-241
© 1994 The Raptor Research Foundation, Inc.
EFFECTS OF RAPTORS ON THE
ACTIVITY OF SANDGROUSE
Peter N. Ferns and Shelley A. Hinsley
School ofPure and Applied Biology, University of Wales, College of Cardiff
P.O. Box 915, Cardiff, CF1 STL, United Kingdom
—
Abstract. Raptors accounted for 55% ofthe disturbance incurred by two species ofsandgrouse (Pterocles
alchata and P. orientalis) in breeding habitat in La Mancha, Spain, during spring and summer. The main
species of raptors were the golden eagle {Aquila chrysaetos) in spring and Montagu’s harrier (Circus
pygargus) in summer. During the breeding season, Montagu’s harriers carried out the majority of their
foraging at those times of day when male sandgrouse collected water for their chicks. This presumably
enabled the harriers to locate chicks more easily. Sandgrouse lost no more than 0.5 hr of feeding time
per day as a result of disturbance by raptors, and yet were still able to rest for at least 3 hr a day at
times when they could have been feeding. Predators as a whole were probably the major factor responsible
for the different dispersion patterns adopted by sandgrouse at different times of year.
Key Words: golden eagle; Montagu’s harrier; Pterocles alchata; Pterocles orientalis; raptor disturbance;
sandgrouse; vigilance.
—
Resumen. -Aves rapaces fueron responsables por 55% de las perturbaciones sufridas por dos especies
de gangas (Pterocles alchata and P. orientalis) en sus areas de reproduccion en la Mancha, Espana, durante
la primavera y el verano. Las especies de aves de rapina mas prominentes fueron el aguila real (Aquila
chrysaetos) en la primavera y el aguilucho cenizo (Circus pygargus) en el verano. Durante la estacion de
reproduccion los aguiluchos cenizos llevaron a cabo la mayoria de su forrajeo en las horas del dia en que
las gangas machos estaban recogiendo agua para sus polios. Esto supuestamente les permitio a los
aguiluchos localizar los polios mas facilmente. Las gangas perdieron no mas de media hora del tiempo
de alimentacion diario debido a las perturbaciones por aves de rapina sin embargo estuvieron inactivos
por lo menos durante 3 horas al dia cuando hubieran podido estar alimentando. Las aves de rapina en
general fueron probablemente el factor mas importante en determinar los distintos partrones de dispersion
de las gangas en distintas epocas del ano.
[Traduccion Autores]
Raptors can exert a significant influence on the rowhawks attacked particular tit (Parus spp.) flocks
flocking behavior of birds, both in the short term about once per day on average, and disturbed the
and the long term. In response to attacks by spar- birds for only about 2 min. Longer disturbances can
rowhawks (Accipiter nisus), redshanks (Tringa tot- be inferred from detailed studies ofmerlins and spar-
anus) deserted their winter feeding territories for up rowhawks attacking wintering waders (Page and
to an hour and formed flocks, while turnstones (Ar- Whitacre 1975, Boyce 1985, Cresswell and Whit-
enaria interpres) reduced their individual spacing from field 1994), though no actual estimates are available
three bird lengths to one bird length (Whitfield 1988). in these cases.
Myers (1984) found that winter feeding territories In order to derive estimates of the total time lost
were abandoned by sanderlings (Calidris alba) in by pin-tailed sandgrouse (Pterocles alchata) and black-
years when merlins (Falco columbarius) were more bellied sandgrouse (P. orientalis) as a result of dis-
abundant. These studies focused on the influence of turbance by raptors, we recorded the amount oftime
raptors on the flocking and vigilance of potential the birds spend being vigilant, the number of inci-
prey (see also Metcalfe 1989). The amount of time dents ofdisturbance and the factors responsible. This
for which target organisms are prevented from car- was done as part of the compilation of time and
rying out other activities has seldom been measured. energy budgets for these two species of sandgrouse
In one early study, Morse (1973) found that spar- in Spain.
236
December 1994 Raptor Disturbance of Sandgrouse 237
Study Area and Methods Table 1. Number of raptors observed while compiling
Observations were made in semi-arid farmland in the sandgrouse time budgets.
province of La Mancha, Spain, in spring (29 March to
21 April) and summer (22 July to 10 August) 1986. Number of
Sandgrouse occurred in flocks during the spring, com-
prising on average 26 (SD = 23, N = 31) individuals in Sightings
pin-tailed sandgrouse, with a maximum flock size of 75, OF Each
Most daytime flocks ofblack-bellied sandgrouse contained Raptor
2-7 birds, though slightly larger groups (maximum 28)
SUM-
roosted together overnight. In summer, both species oc-
Spring MER
curred in scattered family groups. When compiling time N = N =
budgets, we scan-sampled bird activity at 1 min intervals.
In both seasons, the day was divided into 10 equal time Raptor Species 23 da 20 da
periods. These time periods were longer in summer (92
Black kite {Milvus migrans) 4
min) than in spring (82 min). The field area and study
Red kite (M. milvus) 6
methods are described elsewhere (Hinsley 1994, Hinsley
and Ferns 1994). A total of over 46 000 bird observations Montagu’s harrier {Circus pygargus) 1 23
were made, involving on average 29 pin-tailed sandgrouse Common buzzard {Buteo buteo) 3 4
and 6 black-bellied sandgrouse each day in spring, and Imperial eagle {Aquila heliaca) 2
family groups of 2-5 pin-tailed sandgrouse in summer (at Golden eagle {A. chrysaetos) 11
least two different families were watched each day, and Booted eagle {Hieraaetus pennatus) 1
over 40 families were studied in total). Every potential Common kestrel {Falco tinnunculus) 1
predator that was observed close enough to the focal birds
Peregrine falcon {Falco peregrinus) 1 2
to disturb them was recorded. If the same source of dis-
turbance caused more than one group of birds to react, Little owl {Athene noctua) 1
this was treated as a single incident, even if the events
occurred a considerable time apart (actual maximum = 7
sandgrouse adopted alert upright postures or
min).
The observations in the spring were undertaken at a crouched close to the ground (these behaviors were
m
range of 300-1000 (typically 700 m) to avoid disturbing also recorded as vigilance).
the birds, and it was not possible to distinguish gender at The most common identified source ofdisturbance
this time of year. Observations imn the summer were un- was raptors. During a total of 43 d of observations,
dertaken at a range of 200-800 (typically 400 m). In
the summer, groups split up into family parties in which the number of disturbances due to raptors was 60.
A
males, females and juveniles (1-2-mon-old at the time of further 49 disturbances were attributed to other
the observations) were distinguishable. causes. The species of raptors involved (Table 1)
To estimate the amount of time that sandgrouse spent differed significantly between spring and summer,
being vigilant when disturbed, we performed stepwise re-
though the number of incidents was very similar.
gression on the percentage of time spent being vigilant
(arcsine transformed) in each of the 10 daily time periods The most important species in spring was the golden
against the number of records of disturbance caused by eagle. The most important species in summer was
all factors that wewere able toidentify. These were divided Montagu’s harrier (golden eagles were absent from
into five categories comprising the numbers of the most the area during the summer). Because the same in-
common raptors in each season (golden eagle [Aquila chry-
dividual raptor may have caused disturbance on more
saetos] in spring, Montagu’s harrier [Circus pygargus] in
summer); other raptors; other birds (mainly red-legged than one day, there may have been some unavoidable
partridges [Alectoris rufa] and great grey shrikes [Lanius pseudoreplication. However, the proportion of dis-
excubitor]); mammals (rabbits, sheep and dogs); and hu- turbances caused by the most abundant raptor (com-
mans (including noises caused by human activities). This
pared with all other birds) in the two seasons was
method of measuring the affect of disturbance has the
P <
advantage of taking into account losses of feeding time significantly different (Fisher’s exact test, 0.005).
A
through increased alertness even when this lasted for some greater diversity of raptors was thus responsible
time after the disturbance ended. for disturbance during the spring (Table 1). Red
kites (Milvus migrans) and black kites (M. milvus)
Results
were recorded in spring but not summer, both being
A
In the absence of disturbance, sandgrouse vigi- migrants in the area. flock of 106 black kites that
lance was typically restricted to short bouts with the passed over on migration on the evening of 3 August
head up. The duration of these bouts was longest in was not included since it caused no disturbance.
male pin-tailed sandgrouse accompanying chicks (.x Golden eagles were observed circling quite high
= 6.5 sec, SD = 3.9, N = 47). When disturbed, (about 30-50 m) over the fields and hill slopes, and
238 Peter N. Ferns and Shelley A. Hinsley VoL. 28, No. 4
also perched on vantage points overlooking the fields.
Both golden eagles and red kites regularly take birds
the size of adult sandgrouse (Cramp and Simmons
1979). In spring, a juvenile imperial eagle {Aquila
heliaca) took a taxidermic mount of a sandgrouse
(used to measure operative temperature) from the
ground and was only prevented from flying off with
it by the attached data-logger. Montagu’s harriers
and red kites quartered the fields at a height of only
a few meters, and occasionally circled at a greater
elevation. In summer, Montagu’s harriers were ob-
served attacking young sandgrouse, but not adults.
The young of ground-nesting birds form a consid-
erable part of this harrier’s diet during the summer
months (Cramp and Simmons 1979).
budget Vigilance of adult sandgrouse in each time period
(Fig. 1) peaked during the middle of the day in
time spring, and during the morning and evening in sum-
mer. The gap between the summer peaks coincided
with a period of reduced activity of both raptors and
sandgrouse sandgrouse in the middle of the day, induced by high
temperature. The vigilance of juvenile sandgrouse
reached a peak earlier in the morning than that of
of
adults. Adult males spent more time being vigilant
in summer than in spring (19% of daylight hours
percentage compared with 37o), presumably because of the vul-
nerability of their ofTspring during the former pe-
riod. This difference was statistically significant
and H = P =
(Kruskal-Wallis test, \\.?>, 0.001.
While male and female vigilance in the 10 daily
raptors, time periods were correlated significantly during
summer = P < N =
(r 0.69, 0.02, 10), juvenile
of
vigilance was not correlated with that of either par-
ent (r = 0.46 with males and r = 0.12 with females,
Number P >
0.10 in both cases). This is largely because
juveniles spent most of the day feeding or resting,
relying on their parents to act as sentinels (especially
their fathers). They showed most alertness when he
was away collecting water for them during the morn-
ing and when he returned. Juveniles were less sen-
sitive to disturbance by Montagu’s harriers in the
evenings (Fig. 1), probably because the latter spent
most oftheir time hunting rabbits during this period
Figure 1. Selected raptor activity (numbers) and sand-
of the day.
grouse vigilance (percentage of time budget) throughout
Sandgrouse responded to raptors differently in
the daylight period. The total number of raptors is shown
in spring (a), the total number ofraptors other than Mon- spring and summer, and this reflected the different
tagu’s harriers (b), and number of Montagu’s harriers in dispersion pattern they adopted in the breeding sea-
summer son because ofthe vulnerability oftheir young (Lima
(c).
and Dill 1990). In neither spring nor summer did
the sandgrouse form denser groups when disturbed;
if anything the flocks tended to split up. In spring.
December 1994 Raptor Disturbance of Sandgrouse 239
they merely flew to another of the several fields in that caused by humans {t = 4.8, P = 0.003). The
which they normally foraged during the course of larger size of this species apparently made it less
the day. In summer, the first response of both adults vulnerable to disturbance by most raptors.
and young to the presence of a raptor was to crouch
and freeze. If a family group was located by a raptor,
the adults usually mobbed it before flying off, while Discussion
the young ran or flew into the nearest dense vege- While the spring flocks of sandgrouse may have
tation. Consequently, their activity was affected been reasonably easy for golden eagles and other
mainly by a direct loss of time available for feeding, raptors to locate, they did not provide a particularly
We
preening, drinking and other activities. saw three easy target as indicated by the rarity of the attacks
direct attacks by Montagu’s harriers on juvenile we witnessed. Young sandgrouse during the breed-
sandgrouse, all unsuccessful, and adults with young ing season were much more vulnerable, though they
mobbed Montagu’s harriers on four additional oc- must have been more difficult to locate in the well-
casions. Male vigilance showed no significant cor- dispersed family groups. It is thus not surprising
relation with disturbance factors, mainly because that Montagu’s harriers concentrated most of their
they spent such a high proportion of the time alert, foraging activity in the period when male sandgrouse
even in the absence of any obvious disturbance (19% returned with water from the waterholes, since this
of daylight hours spent alert, compared with 12% in provided them with a potential clue to the where-
females). abouts of young. Such predation pressure may have
Since disturbance often caused sandgrouse to fly led in turn (c.f. Lima and Dill 1990) to a wider
in spring, there was significantly more sandgrouse spread ofwatering times amongst males in this area,
alertness in time periods in which the birds spent including evening watering. The two species of
more time flying (r = 0.80, P < 0.01, N ^ 10 in all sandgrouse we observed tended to forage in difTerent
cases). In summer, reduced levels of alertness in both types ofhabitat (Hinsley 1994), but we do not believe
sexes of pin-tailed sandgrouse were associated with that this choice was influenced by raptor activity.
periods of resting (males, r = —0.68, P < 0.05; The pin-tailed sandgrouse was the more vulnerable
females, r = —0.67, P < 0.05). Differences in flock species, but it foraged in open fields where it was
sizes were not significant in different time periods, more visible than on the hill slopes favored by black-
nor was alertness significantly correlated with flock bellied sandgrouse. As suggested by Lima and Dill
size in either species. Therefore, alertness within (1990), vigilance costs were higher during the breed-
seasons was not significantly correlated with any ing season. Adults with young were also prepared
variable that might have confounded its correlation to run greater risks by mobbing raptors during the
with disturbance, including flock size or distance to summer.
cover (Elgar 1989). Predation of young sandgrouse probably repre-
Stepwise regression analysis indicated that a high sents a major source of mortality (Maclean 1985).
proportion of the time spent being vigilant was ac- However, young are likely to be even more vulner-
counted for by the observed sources of disturbance, able to raptors at water holes (Cade 1965) and this
despite the small sample size (TV = 10 in all cases). may explain why males continued to carry water to
In spring, 48% of the variation in pin-tailed sand- them even after they had become capable of flying
grouse vigilance between time periods was accounted to water holes for themselves. Raptors recorded at-
=
for by the overall number of raptors recorded {t tacking or consuming sandgrouse in other areas in-
3.2, P < 0.02) and 20% by human disturbances {t clude pallid harrier (Circus macrourus), golden eagle,
= 2.4, P = 0.05). In summer, 46% offemale vigilance greater kestrel (Falco rupicohides), sooty falcon (F.
was accounted for by raptors other than Montagu’s concolor), lanner falcon (F. biarmicus) and Barbary
harriers {t = 2.5, P < 0.05) and 257o by mammals falcon (F.pelegranoides) (Ali and Ripley 1983, Cramp
{t = 2.5, P < 0.05). Also in summer, 68% ofjuvenile 1985, Urban et al. 1986).
vigilance was accounted for by the number of Mon- The amount oftime males with young spent being
tagu’s harriers {t = 4.1, P = 0.003). Fifty-seven vigilant (19% of daylight hours) was much less than
percent ofblack-bellied sandgrouse vigilance in spring that of male barnacle geese (Branta leucopsis) es-
was accounted for by disturbance caused by dogs, corting their broods (48%) (Forsland 1993). In both
sheep and rabbits (^ = 5.6, P = 0.001), and 19% by species, the males devoted more time to vigilance
240 Peter N. Ferns and Shelley A. Hinsley VoL. 28, No. 4
than the females. Walters (1990) found no difference Acknowledgments
in the response of long-toed lapwings {Vanellus cras- We are most grateful to the Fundacion Jose Maria
sirostris) and blacksmith plovers (V. armatus) ac- Blanc for allowing us to work on their estate in Spain and
cording to whether or not they had young, but they tsoanDd.grHo.usTeh.oSmaAsHfowraslocsautpinpgorstuecdh baygoaodSEplRaCce rteossetaurdchy
did respond more strongly to harriers than to eagles. studentship. We would also like to thank three referees
The intensity of predatory disturbance at our study for numerous improvements to the manuscript.
site was relatively low compared with some others.
For example, at Bolinas Lagoon in California, a
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